Shape information coder and decoder dividing shape information into lower-resolution subsets

ABSTRACT

A shape information coder divides the pixels in the shape information into subsets representing different subsamplings of the shape information. Each pixel in each subset is coded with reference to a context which may be derived from reference pixels in more than one of the subsets. The context preferably includes reference pixels located on all sides of the pixel being coded, which leads to efficient coding. The shape information coder may have two or more cascaded stages, each operating as above, with one subset of pixels passed from each stage to be coded in the next stage. The resulting coded shape information is useful in, for example, wavelet coding of pictures.

BACKGROUND OF THE INVENTION

The present invention relates to the coding of shape information, aprocess carried out in the coding and decoding of still and movingpictures.

When still or moving pictures are coded and decoded, a coding ordecoding operation must sometimes be restricted to the picture elementsin an irregular area. This occurs when hierarchical coding methods suchas wavelet methods are employed. It then becomes necessary to furnishinformation specifying the shape of the area. The shape information canbe represented in the form of a bi-level image, which must also be codedand decoded.

When a picture is coded hierarchically, it is convenient to code theaccompanying shape information hierarchically as well. A known way to dothis is to generate a series of bi-level images of decreasingresolution, and code the difference between each successive pair ofimages in the series by, for example, an entropy coding technique.

Typically, each bi-level image in the series has half the vertical andhorizontal resolution of the preceding image. Thus each picture elementor pixel in one image in the series derives from four pixels in thepreceding image. In FIG. 1, for example, pixel A is derived fromhigher-resolution pixels a, b, c, d by a majority-decision rule. In someconventional coders, to improve the quality of the reduced image, theset of higher-resolution pixels is expanded to include more than fourpixels.

A problem when shape information is coded in this way is that thewavelet transform and other hierarchical coding methods employ adecimating type of subsampling to reduce image resolution. To reduce theresolution by half, for example, they discard every second pixel in thehorizontal and vertical directions. This subsampling method differs fromthe majority-decision methods employed in conventional hierarchicalmethods of coding shape information, leading to inconsistencies betweenthe shape information and picture information. In wavelet coding, theresult may be that the wavelet transform is applied to the wrong pixels,leading to image defects when the coded picture is decoded.

A further problem is that in the decoding process, each pixel can bedecoded only with reference to pixels that have already been decoded. Inconventional coding and decoding methods, the reference pixels aretherefore disposed on only two sides of the pixel being coded ordecoded. Use of this type of reference-pixel context does not lead tocompression ratios as high as could be achieved if a more completecontext were available. Some coders attempt to improve the context byincreasing the number of reference pixels, but the resulting increase inthe number of context states can actually lower the coding compressionratio.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to obtainhierarchically coded and decoded shape information that is consistentwith hierarchically coded and decoded picture information.

Another object of the invention is to improve the compression ratio ofcoded shape information.

The invented shape information coder has resolution converting meansthat processes input shape information by dividing the pixels in theshape information into mutually exclusive subsets, each subsetrepresenting a different subsampling of the input shape information andhaving a lower resolution than the input shape information. The subsetsof pixels are stored in a memory means.

A context generating means selects reference pixels from the memorymeans and generates context information. Preferably, the contextinformation generated for a pixel in one of the subsets is obtained fromreference pixels selected from at least two of the subsets, includingreference pixels disposed on all sides of the pixel. The contextinformation may be simplified by assigning different combinations ofreference-pixel values to the same context-information value. A codingmeans codes the subsets of pixels according to the context information.

The memory means may include a separate memory for each subset ofpixels. The coding means and context generating means may include aseparate coder for each memory, and a separate context generator foreach coder, so that different subsets can be coded concurrently.

A hierarchical shape information coder according to the inventioncomprises a plurality of cascaded stages, each separately structured asdescribed above. As input shape information, each stage (except thefirst) receives one of the subsets of pixels generated in the precedingstage. Context information may be supplied from each stage to thepreceding stage, so that the subset of pixels received from thepreceding stage does not have to be stored in the preceding stage.

The invention also provides a shape information decoder and ahierarchical shape information decoder analogous to the above shapeinformation coder and hierarchical shape information coder, usingsimilar context generating means.

The context generating means may comprise a reference pixel generator, areference pixel position and context simplification memory, and acontext converter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings:

FIG. 1 illustrates a conventional hierarchical coding method;

FIG. 2 is a block diagram of a shape information coder illustrating afirst embodiment of the invention;

FIG. 3 illustrates resolution conversion in the first embodiment;

FIG. 4 illustrates context generation in the first embodiment;

FIG. 5 illustrates a variation of context generation in the firstembodiment;

FIG. 6 is a block diagram of a shape information coder illustrating asecond embodiment of the invention;

FIG. 7 illustrates context generation in the second embodiment;

FIG. 8 is a block diagram of a shape information decoder illustrating athird embodiment of the invention;

FIG. 9 is a block diagram of a shape information decoder illustrating afourth embodiment;

FIG. 10 is a block diagram of one stage in a shape information coderillustrating a fifth embodiment;

FIG. 11 is a block diagram of one stage in a shape information decoderillustrating a sixth embodiment;

FIG. 12 is a block diagram illustrating the internal structure of acontext generator according to the present invention; and

FIG. 13 is a block diagram illustrating the internal structure of aconventional context generator.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described with reference to theattached drawings, in which like parts are indicated by like referencecharacters.

Referring to FIG. 2, the first embodiment is a shape information coderthat receives shape information comprising a set of bi-level pixels S1Oand outputs blocks of coded shape information S15. The coding processproceeds through three resolution-reduction stages, the first stage 1generating subsets of comparatively high-resolution shape information,the second stage 2 generating subsets of medium-resolution shapeinformation, and the third stage 3 generating subsets of low-resolutionshape information. Each stage comprises a context generator (C), aplurality of resolution converters (H, V), and a plurality of memories(M). The subsets of shape information generated in these three stagesare coded by a pixel coder.

The comparatively high-resolution shape information generated in thefirst stage has a resolution equal to half the resolution of the inputshape information S10, horizontally and vertically. Each successivestage reduces the resolution by a further factor of two horizontally andvertically.

In the first stage 1, the input shape information S10 is subsampled by ahorizontal resolution converter (H) 11 that outputs two differentsubsets of pixels S11-1 and S11-2 to respective vertical resolutionconverters (V) 12-1 and 12-2. The first vertical resolution converter12-1 outputs two further subsets of pixels S12-1 and S12-2, writingsubset S12-1 in a first memory (M) 13-1, and subset S12-2 in a secondmemory 13-2. The second vertical resolution converter 12-2 outputs afurther subset of pixels S12-3 to a third memory 13-3, and a stillfurther subset S12-4 to a fourth memory 13-4. These memories 13-1, 13-2,13-3, 13-4 are read by a context generator (C) 14, the pixels read frommemories 13-1, 13-2, 13-3, 13-4 being denoted S13-1, S13-2, S13-3,S13-4, respectively. The pixel information S13-2, S13-3, S13-4 read fromthe second, third, and fourth memories is also supplied to the pixelcoder 15, together with context information S14 from the contextgenerator 14.

The second stage 2 is identical to the first stage 1, except that theinput shape information is the first subset of pixel information S13-1read from the first memory 13-1 in the first stage. The second stage 2comprises resolution converters 21, 22-1, 22-2, memories 23-1, 23-2,23-3, 23-4, and a context generator 24. The pixel coder 15 receivespixel information S23-2, S23-3, S23-4 from the second, third, and fourthmemories 23-2, 23-3, 23-4, and context information S24 from the contextgenerator 24.

The third stage 3 is identical to the first two stages, the input shapeinformation now being the subset of pixel information S23-1 read fromthe first memory 23-1 in the second stage 2. The third stage 3 comprisesresolution converters 31, 32-1, 32-2, memories 33-1, 33-2, 33-3, 33-4,and a context generator 34. The pixel coder 15 receives pixelinformation S33-1, S33-2, S33-3, S33-4 from all four memories in thisstage, and context information S34 from the context generator 34.

The memories (M) comprise, for example, random-access memory (RAM)circuits. The resolution converters (H, V) and context generators (C)comprise, for example, address counters that generate memory addressesand logic circuits that extract and sort pixel values. The pixel coder15 comprises, for example, arithmetic, logic, and memory circuitsspecialized for an entropy coding process such as arithmetic coding. Theresolution converters, context generators, and pixel coder may also beimplemented by programs running on a suitable processor, such as amicroprocessor or digital signal processor.

Next, the operation of the first embodiment will be described.

The horizontal resolution converter 11 sorts the pixels in the inputshape information S10 according to their horizontal position. SubsetS11-1 comprises pixels in even-numbered horizontal positions. SubsetS11-2 comprises pixels in odd-numbered horizontal positions.

The first vertical resolution converter 12-1 subsamples subset S11-1 bysorting the pixels according to their vertical position, taking subsetS12-1 from even vertical positions and subset S12-2 from odd verticalpositions. Similarly, the second vertical resolution converter 12-2takes subset S12-3 from even vertical positions and subset S12-4 fromodd vertical positions in subset S11-2.

A similar subsampling of subset S13-1 is carried out in the second stage2, and a similar subsampling of subset S23-1 is carried out in the thirdstage 3. FIG. 3 illustrates the subsampling operations by using circles,squares, triangles, and diamonds to denote pixels at even and oddhorizontal and vertical positions. As illustrated in FIG. 3, the shapeinformation received by each stage is divided into four mutuallyexclusive subsets of pixels.

When these subsampling operations have been completed, the pixel coder15 begins coding the first low-resolution subset of pixels S33-1, readfrom memory 33-1. The pixel coder 15 scans each row of pixels in thissubset S33-1 from left to right, taking the rows in order from top tobottom, and codes each pixel by an entropy coding method such asarithmetic coding, with reference to a context derived from fourneighboring reference pixels that have already been coded. Thesereference pixels are indicated by circled numerals from one to four incolumn I in FIG. 4, in which the dark dot indicates the pixel beingcoded. The four reference pixels are obtained by context generator 34.

The efficiency of entropy coding depends on the extent to which thevalue of each pixel can be predicted from its context. In general, thevalue of the pixel being coded in column I in FIG. 4 is correlatedsufficiently well with the values of the four reference pixels to enablea fairly high compression ratio to be achieved in the coding process.

Next, the pixel coder 15 codes the subset S33-2 of pixels read from thesecond memory 33-2 in the third stage 3. The pixels available for use ascontext information now include all of the pixels in memory 33-1, aswell as pixels from memory 33-2 that have already been coded. Thecontext generator 34 selects the five reference pixels indicated incolumn II in FIG. 4. In the pixel information S23-1 input to the thirdstage 3, the first two reference pixels (1 and 2), taken from memory33-1, are the pixels immediately above and below the pixel being coded(the dark dot). The third reference pixel (3), taken from memory 33-2,is located two pixels to the left of the pixel being coded. The fourthand fifth reference pixels (4 and 5), taken from memory 33-1, arelocated two pixels to the right of the pixel being coded, in the rowsabove and below the row of the pixel being coded. The pixel coder 15performs entropy coding using this five-pixel context. A very highcompression ratio can be expected, because the context is derived fromreference pixels on all four sides of the pixel being coded.

Next, the pixel coder 15 codes the subset S33-3 of pixels read from thethird memory 33-3 in the third stage 3, using entropy coding with thefive-pixel context indicated in column III in FIG. 4. The first tworeference pixels (1, 2), taken from memory 33-1, are directly adjacentthe pixel being coded, being disposed immediately to its left and rightin the information S23-1 input to the third stage 3. The third referencepixel (3), from memory 33-3, is located two pixels above the pixel beingcoded. The fourth and fifth reference pixels (4, 5), from memory 33-2,are diagonally adjacent the pixel being coded, to the lower left andlower right. A very high compression can again be expected.

Next, the pixel coder 15 codes the subset S33-4 of pixels read from thefourth memory 33-4 in the third stage 3, using entropy coding with thefour-pixel context indicated in column IV in FIG. 4. In the inputinformation S23-1, the first two reference pixels (1, 2), taken frommemory 33-2, are located immediately to the left and right of the pixelbeing coded. The third and fourth reference pixels (3, 4), taken frommemory 33-3 are located immediately above and below the pixel beingcoded. A very high compression ratio can again be expected, as thecontext includes all four immediately adjacent pixels.

The pixel coder 15 has now coded all pixels in the subset S23-1 readfrom the first memory 23-1 in the second stage 2, obtaining four blocksof coded low-resolution shape information. Next, the pixel coder 15codes the subsets of pixels S23-2, S23-3, and S23-4 stored in thesecond, third, and fourth memories in the second stage 2 in the mannerdescribed above, using entropy coding with the contexts illustrated incolumns II, III, and IV, respectively, in FIG. 4. The contextinformation is furnished by context generator 24. This completes thecoding of all pixels in the subset S13-1 read from the first memory 13-1in the first stage, and adds three blocks of coded medium-resolutionshape information to the coded output S15.

Finally, the pixel coder 15 codes the subsets of pixels S13-2, S13-3,and S13-4 stored in the second, third, and fourth memories in the firststage 1 in the same way, again using entropy coding with the contextsillustrated in columns II, III, and IV in FIG. 4, the contextinformation being furnished by context generator 14. Adding three blocksof coded high-resolution shape information to the coded output S15, thisprocess completes the coding of all pixels in the original shapeinformation S10.

The first embodiment codes all of the original shape information S10 inan efficient hierarchical manner, using the same type of subsampling asemployed in hierarchical picture coding methods such as wavelet methods.If the shape information is employed together with wavelet picturecoding, to determine the pixels to which the wavelet transform isapplied, then the coded shape information can be furnished to a waveletdecoder, enabling the decoder to obtain accurate shape information ateach hierarchical stage of the wavelet decoding process. In this case,the first embodiment can be practiced as part of a wavelet picturecoder, sharing the same memory means and resolution conversion means,requiring only the addition of the context generators 14, 24, 34 andpixel coder 15.

The first embodiment also provides a high compression ratio, becausewith the exception of the pixels in subset S33-1, the context of eachpixel is derived from reference pixels disposed on all four sides of thepixel, taken from at least two subsets.

As a variation of the first embodiment, the context generators 14, 24,34 can select contexts as shown in FIG. 5. Columns I and II, used incoding subsets S33-1, S33-2, S23-2, and S13-2, are identical to columnsI and II in FIG. 4, with reference pixels read from the first and secondmemories in each stage. The five-pixel context in column III, used inthe coding of subsets S13-3, S23-3, S33-3, includes reference pixelsfrom the first and third memories in each stage. The six-pixel contextin column IV, used in the coding of pixels read from the fourth memoryin each stage, includes reference pixels from the first and fourthmemories in each stage. The contexts in columns II, III, and IV againsurround the pixel being coded on all four sides. This variation enablesthe subsets of pixels stored in the second, third, and fourth memoriesin each stage to be coded in any order, providing greater flexibility inadapting the shape information coding process to suit, for example, awavelet picture coding process.

As another variation, the number of hierarchical stages can be increasedor decreased. For example, there may be only one stage 1, in which casethe first comparatively high-resolution subset S13-1 in FIG. 2 issupplied to the pixel coder 15. The shape information in this subsetS13-1 can be used in one stage of a wavelet coding or decoding process.The input shape information S10, comprising subset S13-1 combined withthe other three subsets S13-2, S13-3, S13-4, is used in a higher stageof the wavelet coding or decoding process. Thus the shape information isstill coded in a hierarchical manner.

FIG. 6 illustrates a second embodiment of the invention. In the secondembodiment, the subset of pixels S12-1 output by the first verticalresolution converter 12-1 in the first stage 1 is supplied directly tothe horizontal resolution converter 21 in the second stage 2, withoutbeing stored in the first stage 1. Similarly, the subset of pixels S22-1output by the first vertical resolution converter 22-1 in the secondstage 2 is supplied directly to the horizontal resolution converter 31in the third stage 3, without being stored in the second stage 2. Asidefrom these differences, the second embodiment has the same structure asthe first embodiment.

The subsets of low-resolution information S33-1, S33-2, S33-3, and S33-4produced in the third stage 3 are coded as in the first embodiment,using entropy coding with the contexts illustrated in columns I to IV inFIG. 4.

The subsets of medium-resolution information S23-2, S23-3, and S23-4generated in the second stage 2 are also coded as in the firstembodiment, except that since subset S22-1 has not been stored in thesecond stage 2, it cannot be read by context generator 24. Instead, thenecessary reference pixels are obtained by the context generator 34 inthe third stage 3, which reads them from memories 33-1, 33-2, 33-3,33-4.

The storage locations of these reference pixels vary in relation to theposition of the pixel being coded, depending on whether the pixel beingcoded is disposed in an even or odd horizontal position and an even orodd vertical position. FIG. 7 illustrates the various cases for thecoding of pixels in subset S23-2, using the same numbering of referencepixels as in column II of FIG. 4. Column II-a illustrates the context ofa pixel (dark dot) stored in even vertical and horizontal positions inmemory 23-2. Context generator 24 reads one reference pixel (3) frommemory 23-2 in the second stage 2; context generator 34 reads onereference pixel from each of the four memories 33-1, 33-2, 33-3, 33-4 inthe third stage 3. Column II-b similarly illustrates the context of apixel disposed in an even vertical and odd horizontal position. ColumnII-c illustrates the context of a pixel disposed in an odd vertical andeven horizontal position. Column II-d illustrates the context of a pixeldisposed in odd vertical and horizontal positions. In each case, theresulting context is the same as the context in the first embodiment,shown in column II in FIG. 4.

The high-resolution subsets of pixels S13-2, S13-3, and S13-4 are alsocoded in this way, the necessary reference pixels being obtained by allthree context generators 14, 24, 34.

The second embodiment produces the same coded shape information as thefirst embodiment, but requires less memory. The same effects areobtained as in the first embodiment, and the same variations arepossible.

As a third embodiment of the invention, FIG. 8 illustrates a shapeinformation decoder for decoding the coded shape information generatedby the first or second embodiment. The decoder comprises a plurality ofmemories (M), resolution deconverters (H′, V′), and context generators(C) organized into three stages 101, 102, 103, and a pixel decoder 115that receives coded shape information S15. The first or high-resolutionstage 101 comprises a horizontal resolution deconverter (H′) 111, whichreceives subsets of pixels S111-1 and S111-2 from a pair of verticalresolution deconverters (V′) 112-1 and 112-2 and outputs decoded shapeinformation S110. The first vertical resolution deconverter 112-1 readsa subset of pixels S112-1 from a first memory 113-1, and another subsetof pixels S112-2 from a second memory 113-2. The second verticalresolution deconverter 112-1 reads a subset of pixels S112-3 from athird memory 113-3, and another subset of pixels S112-4 from a fourthmemory 113-4. Resolution deconverters 111, 112-1, and 112-2 performoperations reverse to the subsampling operations carried out by theresolution converters 11, 12-1, 12-2 in the first embodiment. Contextgenerator 114 supplies context information S114 obtained from memories113-1, 113-2, 113-3, 113-4 to the pixel decoder 115, operating in thesame way as context generator 14 in the first embodiment. When read bythe context generator 114, the contents of memories 113-1, 113-2, 113-3,113-4 are denoted S113-1, S113-2, S113-3, and S113-4, respectively. Thepixel decoder 115 writes decoded pixels S116 in the second, third, andfourth memories S113-2, S113-3, S113-4.

The second stage 102 is structured similarly, comprising a horizontalresolution deconverter 121, a pair of vertical resolution deconverters122-1 and 122-2, memories 123-1, 123-2, 123-3, 123-4, and a contextgenerator 124. The horizontal resolution deconverter 121 outputs acomparatively high-resolution subset of pixels S117 to the first memory113-1 in the first stage 101. The pixel decoder 115 writes decodedpixels S126 in the second, third, and fourth memories 123-2, 123-3,123-4 in the second stage 102.

The third stage 103 is also similar, comprising a horizontal resolutiondeconverter 131, a pair of vertical resolution deconverters 132-1 and132-2, memories 133-1, 133-2, 133-3, 133-4, and a context generator 134.The horizontal resolution deconverter 131 outputs a medium-resolutionsubset of pixels S127 to the first memory 123-1 in the second stage 102.The pixel decoder 115 writes decoded pixels S136 in the second, third,and fourth memories 133-2, 133-3, 133-4 in the third stage 103, anddecoded pixels S137 in the first memory 133-1 in this stage 103.

Next, the operation of the third embodiment will be described. Duringthis operation, the pixel values stored in each memory are the same asthe pixel values stored in the corresponding memory in the firstembodiment.

Upon receiving coded shape information S15, the pixel decoder 115performs an entropy decoding process such as an arithmetic decodingprocess on the first block of coded low-resolution shape information toobtain decoded shape information S137, which is stored in the firstmemory 133-1 in the third stage. For the decoding of each pixel, thecontext generator 134 supplies a context derived from reference pixelsS133-1 that have already been decoded and stored in memory 133-1. Next,the pixel decoder 115 decodes the second block of coded low-resolutionshape information to obtain the pixels to be stored in the second memory133-2. The context generator 134 supplies context information derivedfrom reference pixels S133-1 and S133-2 read from memories 133-1 and133-2. The third and fourth blocks of low-resolution shape informationare similarly decoded and stored in the third and fourth memories 133-3,133-4. The contexts used are the same as in the first embodiment,illustrated in FIG. 4. When decoding of the low-resolution shapeinformation is completed, the resolution deconverters 132-1, 132-2, 131reassemble the decoded pixels to obtain the medium-resolution subset ofpixels S127, which is stored in the first memory 133-1 in the secondstage 102.

The pixel decoder 115 now decodes three blocks of codedmedium-resolution shape information S15, while context generator 124supplies context information comprising reference pixels S123-1, S123-2,S123-3, S123-4 that have already been decoded. Resolution deconverters122-1, 122-2, 121 reassemble the four medium-resolution subsets ofdecoded pixels S123-1, S123-2, S123-3, S123-4 to obtain thecomparatively high-resolution subset of pixels S117, which is stored inmemory 113-1 in the first stage 101.

Finally, three blocks of coded high-resolution shape information S15 aredecoded and reassembled in the same way to.obtain the decoded shapeinformation S110.

The decoding process can proceed in parallel with a wavelet picturedecoding process, the shape information obtained at each intermediatestage being used to place pixels produced by wavelet decoding in theircorrect positions. Specifically, the low-resolution shape informationS137 decoded first is used in a first stage of the wavelet picturedecoding process. The medium-resolution shape information S127 is usedin a second stage of the wavelet picture decoding process. Thecomparatively high-resolution shape information S117 is used in a thirdstage of the wavelet picture decoding process. The final output shapeinformation S110, which has the highest resolution, is used in a finalstage of the wavelet picture decoding process. The resolutiondeconverters employ the same type of reverse subsampling as employed inwavelet decoding, permitting the sharing of resolution deconvertingmeans between the wavelet decoder and shape information decoder, and ateach stage of the picture decoding process, the wavelet decoder canobtain precisely the shape information it needs for correct picturedecoding.

As a fourth embodiment, FIG. 9 illustrates a decoder similar to thethird embodiment, but lacking the first memories in the first and secondstages 101, 102. The first vertical resolution deconverter 122-1 in thesecond stage 102 receives shape information S122-1 directly from thehorizontal resolution deconverter 131 in the third stage 103. The firstvertical resolution deconverter 112-1 in the first stage 101 receivesshape information S112-1 directly from the horizontal resolutiondeconverter 121 in the second stage 102.

The fourth embodiment operates in the same way as the third embodiment,except that the context generators 114, 124, 134 function as in thesecond embodiment. Thus when decoding medium-resolution shapeinformation S126, the pixel decoder 115 receives context informationS124 and S134 from the context generators 124, 134 in both the secondand third stages 102, 103. Similarly, when decoding high-resolutionshape information S116, the pixel decoder 115 receives contextinformation S114, S124, S134 from the context generators 114, 124, 134in all three stages. The resolution deconverters in all stages operateafter all pixels have been decoded and stored in the memories.

The fourth embodiment provides the same effects as the third embodiment,while using less memory.

Next, a fifth embodiment will be described. The fifth embodiment is acoder comprising cascaded stages with four memories in each stage, as inthe first embodiment. In the fifth embodiment, a separate contextgenerator and pixel coder are provided for each memory, and the codingof the contents of all memories proceeds in parallel.

FIG. 10 shows the structure of a typical stage in the fifth embodiment.The input shape information S310 is the subset of pixels read from thefirst memory in the preceding stage, except in the first stage, in whichthe input shape information comprises all of the pixels input to theshape information coder. The horizontal resolution converter 311 andvertical resolution converters 312-1 and 312-2 operate as described inthe first embodiment, storing subsampled subsets of pixels S312-1,S312-2, S312-3, S312-4 in memories 313-1, 313-2, 313-3, 313-4. The firstof these subsets, read as pixels S313-1 from the first memory 313-1, issupplied to the next stage, and to three context generators 314-2,314-3, 314-4. Pixels S313-2 read from the second memory 313-2 aresupplied to context generator 314-2 and to a pixel coder 315-2. PixelsS313-3 read from the third memory 313-3 are supplied to contextgenerator 314-3 and to another pixel coder 315-3. Pixels S313-4 readfrom the fourth memory 313-4 are supplied to context generator 314-4 andto another pixel coder 315-4. The three pixel coders 315-2, 315-3, 315-4receive respective context information S314-2, S314-3, S314-4 fromcontext generators 314-2, 314-3, 314-4. The contexts are as illustratedin columns II, III, and IV in FIG. 5.

In the lowest-resolution stage, an additional context generator andpixel coder are provided for the first memory. The context shown incolumn I in FIG. 5 is used by this additional context generator andpixel coder.

In the fifth embodiment, as soon as the vertical resolution convertersS312-1, S312-2 have finished storing subsets of pixels in the fourmemories, and the contents of the first memory 313-1 have beentransferred to the next stage, the context generators 314-2, 314-3,314-4 and pixel coders 315-2, 315-3, 315-4 begin coding the contents ofthe second, third, and fourth memories 313-2, 313-3, 313-4, in parallel.The coding process is carried out as described in the first embodiment,using the contexts in FIG. 5 instead of the contexts in FIG. 4.

While this coding process proceeds, the pixels S313-1 transferred to thenext stage are subsampled and stored in similar fashion, and coding inthe next stage begins. Thus coding can proceed concurrently in differentstages, as well as proceeding concurrently on different subsets ofpixels in the same stage.

The fifth embodiment generates the same type of hierarchically codedshape information as in the first and second embodiments, but operatesfaster, because of the concurrent coding of the contents of differentmemories.

In a variation of the fifth embodiment, the same context generators andpixel coder are used in different stages. In this variation, coding inone stage cannot begin until coding in the previous stage has ended, butthe entire coding process can still be speeded up by a factor ofapproximately three, as compared with the first embodiment.

As a sixth embodiment, FIG. 11 shows a typical stage in a decoder fordecoding the coded shape information generated in the fifth embodiment.The typical stage comprises a horizontal resolution deconverter 411,vertical resolution deconverters 412-1 and 412-2, memories 413-1, 413-2,413-3, 413-4, context generators 414-2, 414-3, 414-4, and pixel decoders415-2, 415-3, 415-4.

Briefly, the sixth embodiment operates as follows. Pixels of shapeinformation S416-1 from the next lower-resolution stage are stored inthe first memory 413-1 and supplied (as reference pixels S413-1) to thecontext generators 414-2, 414-3, 414-4. The pixel decoders 415-2, 415-3,415-4 receive coded shape information S415-2, S415-3, S415-4 from anexternal source, and context information S414-2, S414-3,. S414-4from-the context generators, and generate decoded pixels S416-2, S416-3,S416-4, which are stored in the second, third and fourth memories 413-2,413-3, 413-4. The contexts shown in FIG. 5 are used, so that the threepixel decoders 415-2, 415-3, 415-4 can operate concurrently. When thepixel decoders have finished decoding, the resolution deconverters412-1, 412-2, 411 reassemble the decoded data to generate shapeinformation of higher resolution, which is supplied to the nexthigher-resolution stage.

In the lowest-resolution stage, four context generators and four pixeldecoders are provided. All four pixel decoders in this stage can operateconcurrently if the second, third, and fourth pixel decoders lag one rowbehind the first pixel decoder.

The sixth embodiment provides the same effects as the third embodiment,but operates approximately three times as fast, because of theconcurrent operation of the pixel decoders in each stage. A certaindegree of concurrency between different coding stages is also possible,if higher-resolution stages lag slightly behind lower-resolution stages.

As a seventh embodiment, FIG. 12 illustrates a context generator thatcan be applied in any of the preceding embodiments. This contextgenerator differs from conventional context generators by reducing thenumber of context states, to simplify the entropy coding process.

This context generator comprises a reference pixel generator 511 thatreads pixel information S510 from the appropriate memories shown in thepreceding embodiments, and generates discrete context information S511according to pixel position information S512 received from a referencepixel position and context simplification memory 512. A contextconverter 513 converts the discrete context information S511 tosimplified context information S513 according to context conversioninformation S514 received from the reference pixel position and contextsimplification memory 512.

The operation of the seventh embodiment will be described below, takingthe context shown in column II in FIG. 4 as an example. The referencepixel position and context simplification memory 512 stores informationspecifying the positions of the five reference pixels in this context,in relation to the pixel being coded or decoded. The pixel positioninformation S512 instructs the reference pixel generator 511 to obtainthe corresponding pixel values. The discrete context information S511comprises a string of five bits giving the values of the referencepixels numbered one to five in column II in FIG. 4. This context hasthirty-two (2⁵) possible states, from ‘00000’ to ‘11111.’

The context conversion information S514 instructs the context converter513 to reduce these thirty-two possible states to ten states by, forexample, the following rules. If the first two reference pixels(numbered 1 and 2 in FIG. 4) both have the value ‘1,’ the simplifiedcontext information S513 has state nine. If these first two referencepixels both have the value ‘0,’ the simplified context information S513has state eight. If the first two reference pixels have differentvalues, the simplified context information S513 has one of eight statesfrom zero to seven, depending on the eight possible combinations ofvalues of the third, fourth, and fifth reference pixels in the context.

As noted earlier, the first two reference pixels in this context aredirectly adjacent the pixel being coded, above and below it. If thesetwo reference pixels have the same value, there is a high probabilitythat the pixel to be coded also has the same value, regardless of thevalues of the other reference pixels. The ten-state context output bythe context converter 513 is therefore almost as effective for entropycoding as the full thirty-two-state context.

Similar rules for simplifying the other contexts illustrated in FIG. 4,or FIG. 5, are stored in the reference pixel position and contextsimplification memory 512.

For comparison, FIG. 13 shows a conventional context generatorcomprising only a reference pixel generator 521 and a reference pixelposition memory 522. This context generator outputs the full context,e.g., the context with thirty-two states in the example above. Anentropy coder such as an arithmetic coder constructs a probabilitydistribution by assigning probabilities to the occurrence of each valueof the pixel to be coded for each context state. If the number ofcontext states is large, the probability calculations become complex,especially if the entropy coder modifies the probability distributionsadaptively during the coding process, as is often done. Furthermore, itis difficult to determine accurate probabilities for a large number ofstates, because each state occurs only infrequently. The resultinginaccurate probability assignments can lead to low compression ratios.

By discarding unimportant context information and thereby reducing thenumber of context states to which probabilities have to be assigned, theseventh embodiment simplifies and speeds up the entropy coding process.If the pixel coder modifies the probability distributions adaptively,the seventh embodiment can also improve the compression ratio, becauseeach stage occurs more frequently, enabling more accurate probabilitiesto be assigned.

Those skilled in the art will recognize that further variations arepossible within the scope claimed below.

What is claimed is:
 1. A shape information decoder for decoding codedshape information relating to a particular image, the coded shapeinformation including a plurality of mutually exclusive coded subsets ofshape information, each coded subset of shape information having lowerresolution than the coded shape information as a whole, the shapeinformation decoder comprising: decoding means decoding said coded shapeinformation according to context information, thereby generating aplurality of subsets of pixels, said subsets including at least aplurality of first-resolution subsets and a plurality ofsecond-resolution subsets, the first-resolution subsets having a firstresolution, the second-resolution subsets having a second resolution,the second resolution being higher than the first resolution; firstmemory means coupled to said decoding means, storing saidfirst-resolution subsets of pixels; first context generating meanscoupled to said first memory means, selecting, for each pixel in one ofsaid first-resolution subsets, reference pixels already decoded by saiddecoding means, and generating said context information from theselected reference pixels; first resolution deconverting meansreassembling said first-resolution subsets of pixels to generatesecond-resolution shape information having said second resolution;second memory means coupled to said decoding means, storing saidsecond-resolution shape information and said second-resolution subsetsof pixels; context generating means coupled to said second memory means,selecting, for each pixel in one of said second-resolution subsets,reference pixels already decoded by said decoding means or belonging tosaid second-resolution shape information, and generating said contextinformation from the selected reference pixels; and second resolutiondeconverting means reassembling said second-resolution subsets of pixelsand said second-resolution shape information to generatethird-resolution shape information having a third resolution, the thirdresolution being higher than the second resolution.
 2. The shapeinformation decoder of claim 1, wherein for each pixel in one of saidfirst-resolution subsets, said first context generating means selectsreference pixels from at least two of said first-resolution subsets, andfor each pixel in one of said second-resolution subsets, said secondcontext generating means selects reference pixels from at least two ofsaid second-resolution subsets or from said second-resolution shapeinformation and at least one of said second-resolution subsets.
 3. Theshape information decoder of claim 2, wherein for each said pixel insaid one of said first-resolution subsets, said first context generatingmeans selects reference pixels disposed on all sides of said pixel insaid second-resolution shape information and for each said pixel in saidone of said second-resolution subsets, said second context generatingmeans selects reference pixels disposed on all sides of said pixel insaid third-resolution shape information.
 4. The shape informationdecoder of claim 1, wherein said first and second context generatingmeans, in generating said context information, assign at least twodifferent combinations of values of said reference pixels to a singlevalue of said context information.
 5. The shape information decoder ofclaim 1, wherein said decoding means generates, as said subsets, fourfirst-resolution subsets, each having one-half the resolution of saidsecond-resolution shape information horizontally and vertically, andthree second-resolution subsets, each having one-half the resolution ofsaid third-resolution shape information horizontally and vertically. 6.The shape information decoder of claim 5, wherein the second-resolutionshape information is arranged in successive lines of pixels, and saidfour subsets include: a first subset including even-numbered pixels ineven-numbered lines; a second subset including even-numbered pixels inodd-numbered lines; a third subset including odd-numbered pixels ineven-numbered lines; and a fourth subset including odd-numbered pixelsin odd-numbered lines.
 7. The shape information decoder of claim 1,wherein said memory means comprises a plurality of memories each storinga different subset among said subsets.
 8. The shape information decoderof claim 7, wherein said decoding means comprises a plurality of pixeldecoders, each concurrently generating a different subset among saidsubsets, and said context generating means comprises a plurality ofcontext generators supplying context information to respective pixeldecoders.
 9. A hierarchical shape information decoder comprising aplurality of cascaded stages, from a highest-resolution stage to alowest-resolution stage, each stage among said stages being separatelystructured as described in claim 1, each stage except saidhighest-resolution stage having a next-higher-resolution stage amongsaid stages, the shape information generated by the resolutiondeconverting means in each said stage except said highest-resolutionstage being provided to the next-higher-resolution stage as one subsetamong the subsets in said next-higher-resolution stage.
 10. Thehierarchical shape information decoder of claim 9, wherein the contextgenerating means in each said stage except said highest-resolution stageprovides context information to the decoding means in thenext-higher-resolution stage, and the shape information received by saidnext-higher-resolution stage is not stored in the memory means in saidnext-higher-resolution stage.
 11. The shape information decoder of claim1, wherein each of the first and second resolution deconverting meanscomprises at least one vertical resolution deconverter and at least onehorizontal resolution deconverter.